Wiring a thermostat to switch off a light

I have a bit of strange request and need some help wiring up a water heater thermostat to switch off a light bulb when it gets to a certain temperature as it's being used in a enclosed incubator.

I've included the wiring diagram of the Danfoss ATC thermostat which civers the temperature range I require and the wiring diagram, I need to connect this up to a regular light bulb. I need the thermostat to cut in and switch the light bulb off when the temperature rises above a set temperature and then to switch back on when the temperature drops.

If you are using a heater with a thermostat then all you need to do is get access to the switched live terminal of the stat and wire a lightbulb to that and the neutral, ie in parallel with the element.

How tight do you need the temp control to be? The hysteresis of that type of thermostat is quite wide - I doubt it will give you the control you need for an incubator. Why not get a thermostat designed for the job?

If you are using a heater with a thermostat then all you need to do is get access to the switched live terminal of the stat and wire a lightbulb to that and the neutral, ie in parallel with the element.

Click to expand...

What? OP, don't try this. All a thermostat in parallel with the element will do is take out the circuit protective device every time the contacts close, along with perhaps destroying the thermostat too.

OP, I admire your efforts to save money, but as a minimum you really need to buy yourself a proper heating thermostat with anticipator. As davelx already pointed out, there will likely be a rather large dead band with the type of stat you've pictured, so it wont really be a lot of use in an incubator.

If you really must use the thermostat you have purchased (and I think you'll be wasting your time if you do, but hey, it's your time!) then you should be using the com and NC terminals, which break on temp rise.

You'll need to come in from a 3A fused plugtop with Live/Brown into terminal 1. You will have to put Neutral/blue into a piece of 5A terminal block. The outgoing cable to the lamp will need Live/Brown to be in terminal 2, and Neutral/Blue joined to the incoming neutral in the aforementioned 5A terminal block.

Green/yellow from the incoming cable MUST go to the earth terminal on the stat, along with the green/yellow cable to the lamp (if an earth is required by the lampholder, that is).

If this 'incubator' needs a constant temperature level - which has to be by the heat from a lamp (rather than a heat mat) then the obvious thing to do is to fit a properly sized lamp which would then have no need of thermostatic control.

There's no point to using an over-sized lamp which has to be regulated, when you can calculate the correct lamp size by using a simple mercury thermometer within the enclosure.

If the enclosure is very small, then I'd say that a 15W pygmy bulb would be adequate. This could be changed for a slightly higher wattage during the winter season, depending on the temperature of the room.

I did build something similar for a project. The PLC would turn the heater on for 10 seconds and off for 90 seconds then on for 11 seconds and off for 89 seconds this would continue until the thermostat opened then process would reverse and it would decrease on time and increase off time by one second each cycle until the thermostat switched state again. Although this worked quite well at keeping temperature constant the chart recorder still produced a sine wave which would vary between peaks and troughs and by changing the size of the controlled area and the mark + space total would vary but with careful changing we could get it to hold at around 0.2 degrees.

However the £100 for the PLC means in real terms it would be far cheaper to buy the right device to start with.

A room thermostat of the electronic type is rated at 0.5 degrees between on and off and a mechanical with heater about 1 degree without heater connected more like 3 degrees and if measured near to heat source rather than at thermostat then even the electronic type rated at 0.5 degrees will allow around a 10 degree differential.

The correct method would be to use a sensor and controller and a controller like this will cost around £90 and you will also need a sensor like this at another £40 and then one starts to realise how cheap the ready built units are.

You may be able to persuade a cylinder 'stat to give reasonably consistent results despite it's sluggish response (necessary in a water heating system to stop it switching the boiler on and off every few seconds) by building lots of thermal inertia into the system.

How about two seperate chambers - a heating chamber containing your heat source (light bulb) and thermostat, seperated from the incubator chamber by a large heat sink.

That way, the wide temperature fluctuations in the heating chamber could be absorbed by the heat sink, which would radiate it's output into the incubator chamber at a fairly constant rate.

What to use for a heat sink? How about a concrete paving slab for the top of the heating chamber / floor of the incubator?

A downside is it would probably take a long time, maybe days, for the temperature of the incubator chamber to stabilise.

When I did stress relief and pre-heating on welded pipes I found a problem with pre-heating because as the welder started to weld near the sensor it would switch off the mats and as he left the area the mats could not heat up fast enough.

As a result I started to use the simmerstat controls instead and was quite surprised how well this simple device could control the temperature. These simple energy regulators worked very well and even when left overnight the chart recorder showed the pipe had held at the 150 degs plus or minus 5 degs all night.

I don't know what temperature is required and what the tolerance is? I have read with alligator eggs the temperature determines the sex. But there are easy ways of controlling the temperature bulbs in series for example.

To have a thermostat to switch off should it creep over the temperature required would make sense used more as a emergency over temperature cut out than a control.

the set up shown here would by selecting different bulb sizes allow one to adjust it to give around the right amount of heat and the thermostat would be set really just in case.

If you are using a heater with a thermostat then all you need to do is get access to the switched live terminal of the stat and wire a lightbulb to that and the neutral, ie in parallel with the element.

Click to expand...

What? OP, don't try this. All a thermostat in parallel with the element will do is take out the circuit protective device every time the contacts close, along with perhaps destroying the thermostat too.

Click to expand...

I wrote wire a *lightbulb* in parallel with the element - if the OP wants a pilot/indicator lamp to show when the WHATEVER is up to temp.

I've now wired in the original water heater thermostat and it's working fine to automatically switch off the light at 40c (the upper limit), it is a bit slow at switching it back on as the temp needs to drop 6c before it switches it back on.

I'm trying to get the incubator stable at 38.5c so currently trying different bulbs and have setup a PC fan to help airflow.